Display devices using liquid crystal compounds, which are used widely at present, are driven in a TN (twisted nematic) mode.
Such driving display devices, however, have problems that since the position of the molecule of the liquid crystal compound present in the element must be changed in order to change displayed images, the driving time necessary therefor is prolonged, and also a voltage necessary for changing the position of the molecule of the liquid crystal compound becomes higher and electric power consumption becomes larger.
Differing from switching elements utilizing the TN mode or a STN mode, switching elements comprising ferroelectric liquid crystal compounds are able to function as switching elements only by changing the direction of molecular orientation of the liquid crystal compounds, and hence the switching time required for operating the switching elements is prominently shortened. Further, because a Ps.times.E value obtained from a spontaneous polarization (Ps) of the ferroelectric liquid crystal compound and an intensity of the electric field (E) applied thereto is an effective energy output for changing the direction of molecular orientation of the liquid crystal compound, the electric power consumption required therefor can also be extremely reduced. Such ferroelectric liquid crystal compounds are suitable particularly for use in display devices for moving picture, because they have two stable states depending upon the direction of the applied electric field, namely, bi-stability, and also have very favorable switching threshold value characteristics.
When such ferroelectric liquid crystal compounds are used in optical switching elements or the like, these compounds are required to have various characteristics such that an operating temperature is in the vicinity of or below ordinary temperature, an operating temperature range is broad, a switching speed is high (fast), and a switching threshold value voltage is within an appropriate range. In particular, of these characteristics, the operating temperature range is especially important when the ferroelectric crystal compounds are put into practical use.
However, in ferroelectric liquid crystal compounds known hitherto, the operating temperature range is generally narrow, and even in the case of ferroelectric liquid crystal compounds having a wide operating temperature range, the operating temperature range is in a high temperature region out of room temperature, as described, for example, in a paper by R. B. Meyer et al., "J. de Phys.", Vol. 36, p. L-69 (1975) or in a paper by M. Taguchi and T. Harada, "Proceedings of Eleventh Conference on Liquid Crystal," p. 168 (1985). Thus, no ferroelectric liquid crystal compounds satisfactory from the standpoint of practical use are obtainable yet.